The long term objective of these studies is to identify cellular mechanisms of hypoxic-ischemic fetal cerebral white matter injury and provide clinically-relevant insight into the histopathological substrates of lesions defined by MR imaging. The specific aims are to: 1) employ high-field strength ex vivo MR imaging to determine the distribution of chronic white matter lesions and use cell-specific and cell death markers to determine the relative sensitivity and specificity of contrast and diffusion-weighted MR imaging;2) elucidate specific molecular mechanisms by which chronic demyelination and oligodendrocyte maturation arrest occur in a slice-culture model enriched in vulnerable white matter;and 3) determine if chronic white matter injury results in a persistent state of vulnerability to ischemia, which is needed to establish better clinical care for critically ill preterm infants at risk for multiple episodes of ischemia and hypotension. To achieve these objectives an in utero preparation in the 0.65 gestation sheep will be utilized in which ischemia will be induced in fetuses through the inflation of occluders placed on the bilateral carotid arteries. Acquisition of MRI data, using a Bruker Biospin small animal imaging system that is equipped with an ultra- high field (12 T) magnet, will include T2 and diffusion anisotropy measures to assess the cellular etiology of lesions shown on MRI. Tissue sections will be stained immunofluorescently for major cell types found to be enriched in the lesions in preliminary studies (oligodendrocytes, astrocytes, macrophages, microglia, and degenerating cells) and then co-registered with lesions identified with MR imaging. Organotypic slice culture will be used to study the effect of specific molecular interactions on the developing and injured white matter. Chronic injured white matter represents a potentially clinically important state whereby preterm infants that suffer cerebral injury remain susceptible to injury over an extended developmental window and may even acquire increased susceptibility to recurrent injury. We will determine if recurrent ischemia imparts increased susceptibility or resistance on glia and axons in our ovine model of white matter injury. PUBLIC HEALTH RELEVANCE: Premature birth is a common clinical problem that results in deleterious consequences for motor and cognitive development resulting in cerebral palsy and related learning disabilities by largely unknown pathogenic mechanisms. Upon completion of this project, imaging, cellular and molecular analysis of chronic white lesions will be combined in a translationally-relevant model. We expect to gain fundamental new insights into pathogenetic mechanisms that trigger disturbances in myelination and result in permanent neurological disabilities.